This invention relates to a method for forming compacts used for materials of sintered products by compressing a law material powder in the powder metallurgical fields. Specifically, this invention relates to a method advantageously used in the case that the sintered products are hollow members such as a pipe or a nut having a through hole, alternatively, the products have one or more special through hole(s), abutment(s), groove(s), or the like. Wherein, the hole is formed in a direction crossing the compressing direction in forming the compact.
When a compact with a pipe-shaped cylindrical body is formed, in the past, powder is filled in a cavity of dies in which a core rod with cylindrical shape is located up and down so as to form a hole in the pipe. Then, an upper punch and a lower punch compress the powder in the direction toward the axis of the pipe. In this case, the direction of the compression by the upper and lower punches is parallel with the hole in the pipe. However, such an ordinary method poses problems of low loading properties of powder. For example, the loading density of powder in dies cannot be stable, and the length of the product is restricted since the loading depth of the powder cannot be sufficiently kept. Moreover, the method clearly poses problem of so called neutral zone. That is, the green density at the center portion (neutral zone) between the upper and lower punches becomes the lowest, thereby sometimes leading to low quality. Furthermore, the compact cannot be easily taken out from the die, and cannot be easily handled. These problems become more serious as the length in the direction of the compression is long.
In order to solve the above problems, it has been proposed to change the direction of the compression in the case that the pipe such as above is formed. That is, the direction of compression is set in the direction perpendicular to the hole of the pipe, namely the direction of the diameter of the pipe. For example, the applicant has been proposed in Japanese Patent Unexamined Publication (Kokai) No. 4-327398, inserting a core rod into a lateral hole formed in a die, thereby traversing a cavity, and compressing powder filled in the cavity by an upper punch and a lower punch, thereby obtaining a compact. According to the proposed method, the lording depth of the powder can be shallow, so that the loading properties of powder improves and the loading density becomes approximately even over the upper and lower zone, thereby inhibiting occurrence of a neutral zone as low as possible. Moreover, the compacts can be easily drawn out from the die and easily handled. In addition, in the case that a screw or a gear is formed on the inner surface of the hole, the green density of that portion can be sufficiently kept.
According to the method for forming compacts described in the above official gazette, the core rod traverses the cavity and the both ends thereof are inserted in the lateral holes of the die in compressing the powder. Therefore, the core rod is exerted with load that bends the rod. When the load is large, the core rod may transform, or break in an extreme case. The official gazette discloses locating the core rod at the neutral zone of the powder to avoid the above-mentioned trouble. However, the location of the core rod cannot easily controlled, whereby the technique cannot be practically used. Moreover a pipe or a nut illustrated in the gazette can be formed through locating the core rod at the neutral zone by crossing the core rod perpendicularly to the compressing direction. However, there is the case that a hole inclines with respect to the compressing direction, or the core rod cannot be located at the neutral zone since the hole is off set from the center portion of the compressing directions. In such cases, the core rod is exerted with load, so that practical use is impossible.
In general, powder in a cavity is loaded with vibration, so that the loading density is made even. However, sometimes the evenness is not sufficient, whereby the compact obtained by compressing the powder with uneven loading density is uneven in the green density, so that products having sufficient qualities cannot be obtained. More specifically, in the case that the product has a thin portion, it is difficult to make even the loading density of the powder. Therefore, improvement for good qualities is desired. In addition, the core rod is drawn out from the die when the compact is in the die, then the die is downwardly moved so as to draw out the compact. At that time, the compact is rubbed with the die, so that there is the possibility of occurrence of damages such as breaking or crumbling. Such a trouble tends to occur remarkably when the products have thin portions, namely the differential of the outer diameter and the inner diameter of the product is small.